The major function of the colon is conservation of water and electrolytes. To accomplish this end, sodium and chloride are actively absorbed by transport proteins on the colonic surface cell luminal membrane. Many of these proteins utilize the lumen to cytoplasm sodium gradient as energy source for uphill (secondary active) transport of nutrients into the cell. In the distal colon, this sodium gradient is reduced by avid sodium absorption in proximal segments. It is possible that in this distal segment, sodium and chloride are absorbed by a novel mechanism based on a recently discovered potassium-activated proton pump found in distal colon. It is thus important to define the role of this proton- potassium pump in colonic electrolyte transport. The colonic pump may mediate transcellular potassium absorption, or may be an important mechanism for the maintenance of intracellular pH. To evaluate these possibilities, studies will be done with isolated colonic surface cells and membrane vesicles, followed by preliminary purification of the pump protein. Intracellularly trapped pH-sensitive dyes will be used to study modulation of intracellular pH by plasma membrane transport proteins in isolated colonic surface cells, with particular emphasis on evaluating the role of a proton-potassium pump in intracellular pH regulation. Using native membrane vesicles, the colonic pump proton pump will be further characterized in terms of electrogenicity, inhibitor sensitivity, and ability to be phosphorylated from ATP, and compared to the closely related proton-potassium ATPase of gastric mucosa. The pump will also be purified by extraction with low concentrations of detergent followed by chromatography. Reconstitution of proton transport activity will be accomplished by incorportion of solubilized material into phospholipid liposomes. Study of transport proteins has given new insight into the pathophysiology of diseases such as cystic fibrosis and peptic ulcers. It is hoped that detailed knowledge of colon transport proteins may enhance our understanding of intestinal diseases of uncertain etiology such as ulcerative colitis, so that more effective therapies can be devised.